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Difference between revisions of "cpp/atomic/atomic flag test and set"

From cppreference.com
< cpp‎ | atomic
(This example in atomic_flag is actually a test_and_set example, so it belongs here)
(Splitted the volatile overloads.)
 
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{{cpp/title | atomic_flag_test_and_set|atomic_flag_test_and_set_explicit}}
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{{cpp/title|atomic_flag_test_and_set|atomic_flag_test_and_set_explicit}}
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{{cpp/atomic/sidebar}}
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{{cpp/thread/navbar}}
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{{ddcl list begin}}
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{{dcl begin}}
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{{ddcl list header | atomic }}
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{{dcl header|atomic}}
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{{ddcl list item | num=1 | notes={{mark since c++11}} | 1=
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{{dcl|num=1|since=c++11|
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bool atomic_flag_test_and_set( volatile std::atomic_flag* p );
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bool atomic_flag_test_and_set( volatile std::atomic_flag* obj ) noexcept;
 
}}
 
}}
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{{ddcl list item | num=2 | notes={{mark since c++11}} | 1=
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{{dcl|num=2|since=c++11|
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bool atomic_flag_test_and_set( std::atomic_flag* p );
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bool atomic_flag_test_and_set( std::atomic_flag* obj ) noexcept;
 
}}
 
}}
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{{ddcl list item | num=3 | notes={{mark since c++11}} | 1=
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{{dcl|num=3|since=c++11|
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bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* p,  
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bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* obj,  
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                                         std::memory_order order );
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                                         std::memory_order order ) noexcept;
 
}}
 
}}
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{{ddcl list item | num=4 | notes={{mark since c++11}} | 1=
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{{dcl|num=4|since=c++11|
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bool atomic_flag_test_and_set_explicit( std::atomic_flag* p,  
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bool atomic_flag_test_and_set_explicit( std::atomic_flag* obj,  
−
                                         std::memory_order order );
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                                         std::memory_order order ) noexcept;
 
}}
 
}}
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{{ddcl list end}}
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{{dcl end}}
  
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Atomically changes the state of a {{cpp|std::atomic_flag}} pointed to by {{tt|p}} to set ({{cpp|true}}) and returns the value it held before.  
+
Atomically changes the state of a {{lc|std::atomic_flag}} pointed to by {{c|obj}} to set ({{c|true}}) and returns the value it held before.
 +
@1,2@ The memory synchronization order is {{c|std::memory_order_seq_cst}}.
 +
@3,4@ The memory synchronization order is {{c|order}}.
  
 
===Parameters===
 
===Parameters===
−
 
+
{{par begin}}
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{{param list begin}}
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{{par|obj|pointer to {{lc|std::atomic_flag}} to access}}
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{{param list item | p | pointer to {{cpp|std::atomic_flag}} to access}}
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{{par|order|the memory synchronization order}}
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{{param list item | order | the memory sycnhronization ordering for this operation }}
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{{par end}}  
−
{{param list end}}  
+
  
 
===Return value===
 
===Return value===
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The value previously held by the flag pointed to by {{tt|p}}
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The value previously held by the flag pointed to by {{c|obj}}.
  
−
===Exceptions===
+
===Notes===
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{{noexcept}}
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{{tt|std::atomic_flag_test_and_set}} and {{tt|std::atomic_flag_test_and_set_explicit}} can be implemented as {{c|obj->test_and_set()}} and {{c|obj->test_and_set(order)}} respectively.
−
 
+
−
===Equivalent definition===
+
−
{{eq fun cpp
+
−
| 1=
+
−
bool atomic_flag_test_and_set( volatile std::atomic_flag* p )
+
−
{
+
−
    return p->test_and_set();
+
−
}
+
−
| 2=
+
−
bool atomic_flag_test_and_set( std::atomic_flag* p )
+
−
{
+
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    return p->test_and_set();
+
−
}
+
−
| 3=
+
−
bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* p,
+
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                                        std::memory_order order )
+
−
{
+
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    return p->test_and_set(order);
+
−
}
+
−
| 4=
+
−
bool atomic_flag_test_and_set_explicit( std::atomic_flag* p,
+
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                                        std::memory_order order )
+
−
{
+
−
    return p->test_and_set(order);
+
−
}
+
−
}}
+
  
 
===Example===
 
===Example===
−
{{example cpp
+
{{example
−
| A spinlock mutex can be implemented in userspace using an atomic_flag
+
|A spinlock mutex can be implemented in userspace using an {{tt|std::atomic_flag}}.
−
| code=
+
|code=
 +
#include <atomic>
 +
#include <iostream>
 
#include <thread>
 
#include <thread>
 
#include <vector>
 
#include <vector>
−
#include <iostream>
 
−
#include <atomic>
 
  
 
std::atomic_flag lock = ATOMIC_FLAG_INIT;
 
std::atomic_flag lock = ATOMIC_FLAG_INIT;
Line 73: Line 48:
 
void f(int n)
 
void f(int n)
 
{
 
{
−
     for(int cnt = 0; cnt < 100; ++cnt) {
+
     for (int cnt = 0; cnt < 100; ++cnt)
−
         while(std::atomic_flag_test_and_set_explicit(&lock, std::memory_order_acquire)) // acquire lock
+
    {
−
            ; // spin
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         while (std::atomic_flag_test_and_set_explicit(&lock, std::memory_order_acquire))
 +
            ; // spin until the lock is acquired
 
         std::cout << "Output from thread " << n << '\n';
 
         std::cout << "Output from thread " << n << '\n';
−
         std::atomic_flag_clear_explicit(&lock, std::memory_order_release);               // release lock
+
         std::atomic_flag_clear_explicit(&lock, std::memory_order_release);
 
     }
 
     }
 
}
 
}
 +
 
int main()
 
int main()
 
{
 
{
 
     std::vector<std::thread> v;
 
     std::vector<std::thread> v;
−
     for (int n = 0; n < 10; ++n) {
+
     for (int n = 0; n < 10; ++n)
 
         v.emplace_back(f, n);
 
         v.emplace_back(f, n);
−
    }
+
     for (auto& t : v)
−
     for (auto& t : v) {
+
 
         t.join();
 
         t.join();
−
    }
 
 
}
 
}
−
| output=
+
|output=
 
Output from thread 2
 
Output from thread 2
 
Output from thread 6
 
Output from thread 6
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===See also===
 
===See also===
−
{{dcl list begin}}
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{{dsc begin}}
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{{dcl list template | cpp/atomic/dcl list atomic_flag}}
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{{dsc inc|cpp/atomic/dsc atomic_flag}}
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{{dcl list template | cpp/atomic/dcl list atomic_flag_clear}}
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{{dsc inc|cpp/atomic/dsc atomic_flag_clear}}
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{{dcl list template | cpp/atomic/dcl list memory_order}}
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{{dsc inc|cpp/atomic/dsc memory_order}}
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{{dcl list end}}
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{{dsc see c|c/atomic/atomic_flag_test_and_set|atomic_flag_test_and_set|atomic_flag_test_and_set_explicit}}
 +
{{dsc end}}
 +
 
 +
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 22:05, 24 September 2023

 
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atomic_flag_test_and_setatomic_flag_test_and_set_explicit
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(C++20)
(C++20)
 
Defined in header <atomic>
bool atomic_flag_test_and_set( volatile std::atomic_flag* obj ) noexcept;
 (1) (since C++11)
bool atomic_flag_test_and_set( std::atomic_flag* obj ) noexcept;
 (2) (since C++11)
bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* obj,
                                        std::memory_order order ) noexcept;
 (3) (since C++11)
bool atomic_flag_test_and_set_explicit( std::atomic_flag* obj,
                                        std::memory_order order ) noexcept;
 (4) (since C++11)

Atomically changes the state of a std::atomic_flag pointed to by obj to set (true) and returns the value it held before.

1,2) The memory synchronization order is std::memory_order_seq_cst.
3,4) The memory synchronization order is order.

Contents

[edit] Parameters

obj - pointer to std::atomic_flag to access
order - the memory synchronization order

[edit] Return value

The value previously held by the flag pointed to by obj.

[edit] Notes

std::atomic_flag_test_and_set and std::atomic_flag_test_and_set_explicit can be implemented as obj->test_and_set() and obj->test_and_set(order) respectively.

[edit] Example

A spinlock mutex can be implemented in userspace using an std::atomic_flag.

Run this code
#include <atomic>
#include <iostream>
#include <thread>
#include <vector>
 
std::atomic_flag lock = ATOMIC_FLAG_INIT;
 
void f(int n)
{
    for (int cnt = 0; cnt < 100; ++cnt)
    {
        while (std::atomic_flag_test_and_set_explicit(&lock, std::memory_order_acquire))
            ; // spin until the lock is acquired
        std::cout << "Output from thread " << n << '\n';
        std::atomic_flag_clear_explicit(&lock, std::memory_order_release);
    }
}
 
int main()
{
    std::vector<std::thread> v;
    for (int n = 0; n < 10; ++n)
        v.emplace_back(f, n);
    for (auto& t : v)
        t.join();
}

Output: 

Output from thread 2
Output from thread 6
Output from thread 7
...<exactly 1000 lines>...

[edit] See also

(C++11)
 the lock-free boolean atomic type
(class) [edit]
(C++11)(C++11)
 atomically sets the value of the flag to false
(function) [edit]
(C++11)
 defines memory ordering constraints for the given atomic operation
(enum) [edit]
C documentation for atomic_flag_test_and_set, atomic_flag_test_and_set_explicit
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